A wide variety of roofing materials has been developed over the years to provide lasting protection of a building. While most of the available alternatives have provided adequate protection of buildings, a common characteristic with most available materials is that they are impervious to precipitation, the intent being to prevent precipitation from penetrating the roofing surface and impacting the underlying structure. However, one consequence from a stormwater management perspective has been a steady increase in impervious surfaces associated with the footprint of most new developments. The introduction of impervious surfaces translates into increased volumes of stormwater runoff and increased rates at which this runoff leaves a given site. In predeveloped conditions (i.e., before a structure and roof), there are typically trees, grasses, shrubs, and other natural ground covers that allow for infiltration, evapotranspiration, and a generally slow rate of surface runoff from precipitation. New structures and the impervious rooftops change the hydrology of a site considerably by creating a fast and efficient path for precipitation to leave the structure and, ultimately, the property.
This widespread change of land cover and associated change in surface runoff hydrology has led to similarly widespread problems with stormwater management throughout the country and world. In watersheds drained by open channels, the increased rate and volume of stormwater runoff can cause flooding and erosion in downstream ditches, streams, and rivers. In man-made systems, the change in hydrology can cause overloaded conveyance systems, property damage, and other related consequences. As a result, many jurisdictions in the United States and worldwide have developed stormwater regulations that, among other things, set regulatory limitations on the rate at which stormwater runoff can leave a site. Typical approaches to achieving these regulations include providing stormwater detention or retention systems to capture and temporarily store stormwater runoff and release it at a slower rate or infiltrate it into surrounding soils.